1. Technical Field
The present invention relates to a surface acoustic wave filter, and in particular, to a narrow-band surface acoustic wave filter which is reduced in size.
2. Related Art
In recent years, a surface acoustic wave filter (SAW filter) has been widely used in telecommunications field. In other words, because of its excellent features such as high performance, small size, mass production, and the like, the SAW filter is frequently used in a mobile phone or the like. FIG. 9 is a plan view illustrating a construction of a cascaded dual mode SAW filter which is constructed by cascading two primary-tertiary longitudinally-coupled dual mode SAW filters (hereinafter, referred to as the dual mode SAW filter). The cascaded dual mode SAW filter is used in an RF filter of a mobile phone or the like. On the main surface of a piezoelectric substrate 31, IDT electrodes 32, 33, and 34 are disposed adjacent to each other along the propagation direction of a surface wave. On both sides of the IDT electrodes 32, 33, and 34, grating reflectors 35a and 35b (hereinafter, referred to as a reflector) are arranged, thereby forming a first dual mode SAW filter F1. In this case, the IDT electrodes 32, 33, and 34 are respectively formed with a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other.
On the same piezoelectric substrate 31, a second dual mode SAW filter F2 composed of IDT electrodes 32′, 33′, and 34′ and reflectors 35′a and 35′b is formed in the same way as the first dual mode SAW filter F1. The first and second dual mode SAW filter F1 and F2 are cascaded, thereby constructing a cascaded dual mode SAW filter.
FIG. 10 illustrates filter characteristics obtained by simulation using the electrode pattern of the cascaded dual mode SAW filter shown in FIG. 9 in order to design an RF filter (in which the center frequency is 1.57542 MHz, the bandwidth is ±1 MHz, and the terminal impedance is 50 Ω) for a GPS which has recently been used in an in-vehicle telephone or the mobile phone, with the piezoelectric substrate set to a 38.7° Y—XLiTaO3 substrate, the center frequency set to 1.5 GHz, 14.5 pairs of IDT electrodes 33 and 33′, 9.5 pairs of respective IDT electrodes 32, 34, 32′, and 34′, the intersection width set to 35λ (λ is a wavelength of the surface wave), each number of reflectors 35a, 35b, 35′a, and 35′b set to 100, and the electrode film thickness set to 7.7%λ. Within the passband, a large ripple appears since a broad-band electrode pattern is used.
A technique for improving the ripple within the passband has been disclosed in JP-A-4-40705 and JP-A-7-74588. That is, as shown in FIG. 11, IDT electrodes 42, 43, and 44 are disposed adjacent to each other along the propagation direction of a surface wave on the main surface of a piezoelectric substrate 41. Further, on both sides of the IDT electrodes 42, 43, and 44, reflectors 45a and 45b are arranged, thereby forming a first dual mode SAW filter F1. On the same piezoelectric substrate 41, a second dual mode SAW filter F2 including IDT electrodes 42′, 43′, and 44′ and reflectors 45′a and 45′b is formed in the same way as the first dual mode SAW filter F1. The first and second dual mode SAW filters F1 and F2 are cascaded. Further, between the first and second dual mode SAW filters F1 and F2, capacity electrodes 46a and 46b are disposed to be orthogonal to the propagation direction of a surface wave. Lead electrodes extending from one side of each of the comb-shaped electrodes of the capacity electrodes 46a and 46b are respectively connected to lead electrodes which cascade the filters F1 and F2. The other sides of the comb-shaped electrodes are respectively grounded, thereby constructing a narrow-band cascaded dual mode SAW filter. With the capacity electrodes 46a and 46b functioning as capacity elements, the ripple within the band disappears.
FIG. 12 shows filter characteristics obtained by simulation in a state where the constants of the dual mode SAW filter are set to be the same as those shown in FIG. 10 and the capacitance values which are formed by the capacity electrodes 46a and 46b are commonly set to 0.64 pF. As a result, a ripple does not exist within the passband, and the passband becomes flat.
JP-A-2002-353777 has disclosed a filter in which at least a pair of electrode fingers are thinned out from a center IDT electrode of a primary-tertiary longitudinally-coupled dual mode SAW filter and short-circuit-type floating electrodes are then disposed and a filter in which grating reflectors are disposed after thinning out such that the impedance of the filter can be matched with desired impedance by thinning out the electrode fingers.
In the narrow-band cascaded dual mode SAW filter according to the related art as shown in FIG. 11, however, the capacity electrodes for forming capacity elements should be provided between two of the dual mode SAW filters F1 and F2. Therefore, there is a problem that the cascaded dual mode SAW filter is enlarged as much as the space of where the capacity electrodes are provided.